1. Field of the Invention
The present invention relates to a process for producing carbon fibers with high strength and high modulus of elasticity such as Young's modulus from mesophase pitches and also relates to an apparatus suitable for the practice of the process. More specifically, the present invention relates to an excellent economical process for producing high quality carbon fibers from mesophase pitches by melt spinning, wherein the pitch is spun while giving a rotatory motion to the pitch, and to a very simple spinning apparatus used for practice of the process. The characteristic feature of the apparatus of the present invention is that the apparatus contains a plug member having a spiral groove thereon, such as a drill point or a worm gear-like structure and the plug member is positioned within a path of pitch flow near a spinning nozzle hole so as to give a rotatory motion to the pitch.
In the specification, the words "pitch-based carbon fibers" means carbon fibers made from pitches.
Carbon fibers are useful materials, and they are recently attracting attention and gathering concern as an important material of the next generation. The carbon fibers may be classified into two groups: a high performance grade carbon fiber with high strength and high modules of elasticity which is used as composite materials in the fabrication of aircraft structures, sports goods, and the like, and a general purpose grade carbon fiber which is mainly used as heat insulator because of its low strength and modulus of elasticity.
2. Description of the Prior Art
High performance grade carbon fibers have been produced mainly by spinning a polyacrylonitrile (PAN) fiber, converting the PAN fiber to infusible state under oxidizing conditions, and subsequently carbonizing or graphitizing it under an inert atmosphere. In contrast to these PAN-based carbon fibers, pitch-based carbon fibers which are produced from pitches, have been regarded as unsuitable for use as structure materials because of their low strength and modulus of elasticity than PAN-based carbon fibers.
Recently however, pitch-based carbon fibers recurred attention because of the low cost of the starting material and because high yield are attainable when they are rendered infusible or carbonized. Vigorous studies are currently made concerning the process for producing of high performance carbon fibers from pitches as the starting material. Several processes have been proposed which permit production of pitch-based carbon fibers showing equal properties to those of PAN-based carbon fibers or even showing far superior modulus of elasticity.
Various processes for the production of pitch-based high performance carbon fibers have been elaborated: in one process (Japanese Patent Disclosure No. 196292/1983), pitches for spinning are produced by hydrogenation and subsequent thermal treatment; in another process (Japanese Patent Disclosure No. 113292/1983), pitches for spinning are produced by fractional solvent extraction of pitches and subsequent thermal treatment of specific fractions thus fractionated. In still another process (Japanese Patent Disclosure No. 86717/1983), pitches for spinning are produced by submitting pitches to a thermal treatment for a prolonged period of time at a relatively low temperature. A characteristic feature common to these processes is that all of the pitches for spinning are so-called mesophase pitch, which contain the mesophase showing an optical anisotropy when examined on a polarized light microscope as the main component.
The mesophases described above are liquid crystals and are formed on heating heavy oils, tars or pitches. In the specification, the words "heavy oil" means an oil having high boiling point and high specific gravity. It is considered that these mesophases shown an optical anisotropy because planar aromatic molecules, developed by thermal polymerization, are aligned in a layered structure. When fibers are produced from such mesophase pitches by melt spinning, the planar aromatic molecules are aligned parallel to the fiber axis by the stress exerted on passing through a spinning nozzle hole. This oriented structure is not disturbed and is maintained throughout the states of rendering the fibers infusible and their carbonization. Therefore, the carbon layers in the carbon fibers thus produced are also oriented along the fiber axis. Such highly oriented carbon fibers show high tensile strength, and when they are graphitized, they show high modulus of elasticity which is not attainable with PAN-based carbon fibers.
In order to improve the performance of pitch-based carbon fibers, it is essential to produce and to use mesophase pitches which will permit formation of well-oriented planar aromatic molecules during the spinning stage, and most of the prior proposals on the processes for the production of high performance carbon fibers relate to processes for the production of high quality mesophase pitches.